1. Field of the Invention
This invention relates to nuclear reactors, and more particularly to apparatus for aligning and maintaining torsional and lateral stability among the reactor vessel and internals structures.
2. Description of the Prior Art
A typical nuclear reactor includes a reactor vessel, a reactor vessel head sealingly engaging the vessel, and reactor internals structures which support a nuclear core within the vessel and align and guide reactor components, instrumentation, and coolant flow. In many reactors the internals include a core barrel supported from the vessel, which barrel radially surrounds the core and is affixed to a lower core support structure. Similarly, an upper core support structure, supported by the reactor vessel and/or the vessel head is positioned above the reactor core to align core components, guide and position instrumentation, and provide top support for core components thereby alleviating undue upward motion. Fine tolerances must be maintained among the various reactor components, to provide, for example, proper lateral alignment and torsional and lateral stability to alleviate undue stress on the components such as the upper support structure. In the prior art, such functions have typically been performed by several pins affixed to a flange of the core barrel, each of which pins both upwardly engage aligned openings in the upper support structure and vessel head, and downwardly engage the reactor vessel.
This pin arrangement, while providing alignment when the reactor is completely assembled, provides no such alignment during disassembly for maintenance or refueling. In most reactors, refueling or maintenance operations are initiated by serially removing the vessel head and then the upper core support structure, thereby rendering the core accessible. More advanced reactor designs, however, provide for simultaneous removal of the vessel head and upper support structure. This type of reactor is described in U.S. Pat. No. 3,836,429, in the name of Erling Frisch et al. With such a reactor, it can be seen that the prior art pin arrangement provides no torsional or lateral stability between the vessel head and upper support structure during the simultaneous removal, as the pins remain with the core barrel. This deficiency could lead to undesirable rocking or pendulum action during movement of the head and upper support structure, as well as misalignment and increased stress during the period of time these structures are removed from the vessel.
It is therefore desirable to provide apparatus which will maintain relative component alignment and stability not only during reactor operation, but also during disassembly. Such apparatus should ideally be compatible with existing reactor designs, add minimally to manufacturing processes and costs, and not add any steps or additional time to the disassembly process.